WASHINGTON (CNN) – A nightmare scenario for military and intelligence officials. ISIS in possession of a nuclear bomb.

The idea may be seem farfetched to you but it’s taken very seriously by the U.S. government. The terror group has already used chemical weapons and expressed a desire to go nuclear.

It’s a fear President Obama has discussed openly and plans to at a nuclear security summit in Washington Thursday.

Raiding the home of a suspected planner of last November’s Paris attacks, Belgian authorities found surveillance video of a top Belgian nuclear scientist. That suspect, part of the same ISIS cell accused of last week’s attacks in Belgium. The shocking discovery turned the heads of counter-terrorism experts who fear that Belgium, with several previous nuclear breaches, could be at risk for terrorists to obtain radiological materials for a so-called dirty bomb.

“A small dirty bomb would not just cause panic, it would not just cause people to flee the city, it would contaminate tens of square blocks for years making them uninhabitable,” said Joseph Cirincione, President of Ploughshares Fund.

Those fears now top the agenda at this week’s nuclear summit. President Obama first convened the gathering of world leaders 6 years ago issuing a call to action.

“It is increasingly clear that the danger of nuclear terrorism is one of the greatest threats to global security,” said Obama.

Since that warning, 12 countries have eliminated nuclear material but tons of unsecured weapons-grade material remains in 25 countries.

ISIS, barely on the radar at the time of the first summit, is now a global network already using chemical weapons on the battlefield.

A recent Harvard University report warned that despite modest improvements in nuclear security, the capability of groups, especially ISIS has “grown dramatically,” suggesting overall “the risk of nuclear terrorism may have increased.”

“We don’t know what the terrorist threat is going to look like 2 year, 5 years, 10 years from now. And to me that’s even stronger reason to lock down all the ingredients of a potential nuclear recipe,” said Matthew Bunn of Harvard University’s Belfer Center.